1. Field of the Invention
This invention relates to a hybrid type rock bit.
More particularly, this invention relates to a roller cone type rock bit wherein one leg of a three leg rock bit is converted to a diamond studded drag bit segment, the remaining two legs support roller cone cutters.
2. Description of the Prior Art
U.S. Pat. No. 4,006,788, assigned to the same assignee as the present invention, describes a rock bit for recovering core samples as well as rock bit variations for drilling oil wells or the like. In each of the several embodiments described, diamond cutters are strategically mounted on the bit body for cutting rock by a shearing action. Each diamond cutter is in the form of a thin diamond plate bonded to a tungsten carbide stud that is inserted into the bit body. Means are also provided for limiting the depth of penetration of the diamond cutters into the rock formation being drilled. For example, rolling cone cutters with a plurality of tungsten carbide inserts protruding from their surfaces limit penetration of the diamond cutters. The protrusion of the carbide inserts from the surface of the cutter cones is less than the length of the diamond cutting face. Typically the diamond cutters are mounted for cutting one portion of the borehole area by shearing action and the rolling cone cutters are mounted for cutting another portion of its area by a chipping and crushing action.
The foregoing patent is disadvantaged in that the multiplicity of diamond cutters strategically placed on the various rock bit embodiments are inadequately cooled and cleaned during the borehole forming operation. Without adequate flow of fluid around and over each of the diamond inserts, the drag bit portion of the hybrid rock bits described may ball up, especially when they are passing through softer formations. In addition, with inadequate flow of fluid around the diamond inserts, the inserts become ineffective and may become overheated and damaged due to lack of cleaning and cooling.
The present invention teaches a means to increase the rate of penetration by utilizing a hybrid rock bit whereby one of the legs of a three leg bit is converted to a drag bit portion, the remaining two legs support conventional roller cones. The drag bit leg has a multiplicity of diamond cutters inserted in the face of the drag bit. Each of the diamond cutters are supplied with hydraulic passages for continuous cooling and cleaning of the cutters.
The diamond inserts may be so strategically positioned to cut the "hills" or high points in the hole bottom that are left uncut by the teeth in the two adjacent roller cones.